storage: Exclude subtransaction detector from coverage.

caucase/storage.py

@@ -44,7 +44,7 @@ class NoReentryConnection(sqlite3.Connection):
   __entered = False
 
   def __enter__(self):
-    if self.__entered:
+    if self.__entered: # pragma: no cover
       raise Exception('Subtransactions are not supported')
     self.__entered = True
     return super(NoReentryConnection, self).__enter__()